Demonstrators torched parked cars and smashed bank windows during a protest against the opening of Milan's Expo 2015.

Thick clouds of smoke from burning cars filled parts of the city centre as groups of protesters, their faces masked against the fumes, threw stones and faced off against lines of police in riot gear.

Police responded with tear gas and water cannons to try to scatter the violent protesters. Hooded protesters tossed bottles and incendiary devices at police and journalists, set rubbish bins on fire and smashed pavements. Firefighters worked to extinguish the blazes.

Milan Expo 2015 May Day
A man attempts to save his car after it was set on fire by protesters during a rally against Expo 2015 in MilanStefano Rellandini/Reuters
Milan Expo 2015 May Day
A man helps another to save his car after it was set on fire by protestersStefano Rellandini/Reuters
Milan Expo 2015 May Day
Firefighters investigate a car that was set on fire by protesters during a rally against Expo 2015 in MilanStefano Rellandini/Reuters
Milan Expo 2015 May Day
Stefano Rellandini/Reuters
Milan Expo 2015 May Day
Stefano Rellandini/Reuters
Milan Expo 2015 May Day
Stefano Rellandini/Reuters
Milan Expo 2015 May Day
Stefano Rellandini/Reuters
Milan Expo 2015 May Day
Policemen help their colleague whose clothes caught alight after being hit by a Molotov cocktailStefano Rellandini/Reuters

The confrontation came hours after a glitzy opening ceremony at the Expo site where Italy Prime Minister Matteo Renzi hailed the start of a six-month-long showpiece of culture and technology, focused on the theme of sustainable food production.

In all, more than 140 countries are taking part in the fair, which follows the 2010 Expo in Shanghai. Cultural events, futuristic architecture, a "supermarket of the future" and dozens of restaurants fill the site, which, critics note, required more than one million square metres of farmland to be concreted over.

Milan Expo 2015 May Day
The "Tree of Life" is seen at Expo 2015 in MilanAlessandro Garofalo/Reuters
Milan Expo 2015 May Day
The Italian pavilionAlessandro Garofalo/Reuters
Milan Expo 2015 May Day
The German pavilionOlivier Morin/AFP
Milan Expo 2015 May Day
The British pavilionAlessandro Garofalo/Reuters

Thousands of police had been deployed to counter the threat of violence before the event, which Renzi has been counting on to reinforce fragile signs of economic recovery after years of stagnation and recession.

"Today it is as though Italy is embracing the world," he said at the opening. "All you experts who kept saying 'We'll never do it' – this is your answer. I like to think that tomorrow begins today."

Instead, the elegant centre of Milan was transformed into a battleground, with sirens and periodic bangs from flash bombs and firecrackers ringing out against the shouts of protesters.

Milan Expo 2015 May Day
Andreas Solaro/AFP
Milan Expo 2015 May Day
Filippo Monteforte/AFP
Milan Expo 2015 May Day
Filippo Monteforte/AFP
Milan Expo 2015 May Day
Filippo Monteforte/AFP
Milan Expo 2015 May Day
Filippo Monteforte/AFP
Milan Expo 2015 May Day
Andreas Solaro/AFP
Milan Expo 2015 May Day
Andreas Solaro/AFP
Milan Expo 2015 May Day
Police officers arrest a woman during a demonstration against Milan's ExpoAndreas Solaro/AFP
Milan Expo 2015 May Day
Police arrest a May Day protesterAndreas Solaro/AFP
Milan Expo 2015 May Day
Andreas Solaro/AFP